In one aspect, the present invention relates to methods and apparatuses for reducing NOx emissions from burners of the type used in process heater, boilers, and other fired heating systems. More particularly, but not by way of limitation, the present invention relates to achieving low NOx emissions in and with a burner having only one combustion stage.
Many industrial applications require large scale generation of heat from burners for process heaters, boilers, or other fired heating systems. If the burner fuel is thoroughly mixed with air and combustion occurs under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, such as in a high temperature environment, nitrogen present in the combustion air reacts with oxygen to produce nitrogen oxides (NOx). It is well known that, other conditions being equal, NOx production increases as the temperature of the combustion process increases. NOx emissions are generally considered to contribute to ozone depletion and other environmental problems.
Prior to today""s increasing concern over the environmental effects of NOx, emissions, single staged burners were in common use in generally all types of fired heater applications. Prior art single stage burners typically comprise one or more fuel nozzles or distributors positioned inside the burner wall. As compared to typical low NOx, burners now in use, the prior art single stage burners are less expensive, less complex, safer, more stable, and simpler to operate, control, and maintain. Prior art single stage burners also typically provide much broader acceptable operating ranges (turndown ratios). Unfortunately, however, the simpler single stage burners heretofore used in the art produce very high levels of NOx, emissions and are not capable of meeting today""s demanding environmental standards and regulations.
Burners designed for combusting fuel with air in a manner resulting in less NOx emissions are commonly referred to as xe2x80x9clow NOxxe2x80x9d burners. One type of apparatus now used for reducing NOx emissions is a xe2x80x9cstaged airxe2x80x9d burner. Staged air burners operate by dividing the flow of combustion air to create a first combustion zone (wherein the fuel is introduced) having a deficiency of air so as to create a reducing environment that suppresses NOx formation and a second combustion zone wherein the remaining portion of air is introduced and the combustion process is completed.
Another type of low NOx apparatus is a xe2x80x9cstaged fuelxe2x80x9d burner wherein all of the combustion air, but only a portion of the fuel to be burned, are introduced in a first combustion zone. The remaining fuel is introduced into a second combustion zone utilizing the oxygen-rich effluent of the first zone. In such a burner, the excess air in the first zone serves to dilute the fuel, which lowers the temperature of the burning gases and thereby reduces the formation of NOx.
Other low NOX methods and apparatuses recirculate and mix furnace flue gases with fuel/air mixtures to dilute the mixtures and to thereby lower the combustion temperature so that NOx formation is reduced. Flue gases are captured from the furnace space and conducted via pipes, ducts, or passageways to a mixer assembly, typically within the burner housing, where the flue gases are mixed with fuel or with fuel and air. The resulting mixture is then burned.
It will be appreciated that to accomplish the foregoing, each of the conventional types of low NOx burners must be rather complex in structure and operation. As compared to high NOx prior art single stage burners, conventional low NOx systems must provide for and include additional hardware, conduits, passageways and other structures to achieve staged introduction of fuel or air, to allow for the burning of fuel/air mixtures in multiple combustion zones, and/or to accommodate the recirculation of furnace gases. This increased level of complexity does not lend itself to low-cost manufacture, reliability, or ease of maintenance. Moreover, in staged burner systems, the necessity of splitting flows to and balancing the performance of multiple combustion zones/stages increases the difficulty of achieving and maintaining operational stability and greatly reduces the available operating range (turndown ratio) of the burner.
One type of low NOx staged fuel burner heretofore known in the art is described in U.S. Pat. No. 5,195,884. In the ""884 burner, a primary portion (preferably about 75%) of the fuel gas used in the burner must be burned in a first (xe2x80x9cprimaryxe2x80x9d) combustion zone within and surrounded by the burner wall. The primary fuel is mixed with air and discharged into the primary combustion zone using one or more mixing and discharge assemblies which project into and are at least partially contained within the throat of the burner. Each of these assemblies comprises a Venturi aspirating tube having a primary fuel gas nozzle positioned at the lower end thereof.
The remaining (secondary) fuel gas used in the ""884 burner is delivered to a secondary combustion zone by four secondary fuel gas nozzles outside of the burner wall. Each of the secondary fuel nozzles has an array of multiple flow ports provided therein which must spread the secondary flue gas in a fan-type pattern covering essentially one-quarter of the exterior of the burner wall. The burner wall has an exterior frusto-conical surface which is contacted by the secondary fuel gas as it spreads outwardly and moves upwardly to the secondary combustion zone. Such contact is said to promote the mixing of internal flue gases with the secondary fuel gas.
Unfortunately, the primary combustion stage alone of the burner described in the ""884 patent produces more NOx emissions than are allowable, for example, under Texas Gulf Coast restrictions and other regulatory requirements. Moreover, without using the primary combustion stage, it has not been possible heretofore to obtain adequate heating or to achieve and maintain stable burner operation.
Like the stage fuel burner described in the ""884 patent, few, if any, of the other low NOx burners presently available are capable of meeting Texas Gulf Coast requirements and other increasingly stringent air quality standards. If suitable new burner technologies capable of satisfying these requirements are not found, the industry will be required to use more expensive and elaborate techniques, such as catalytic reduction, to reduce NOx emissions.
Thus, a need exists for a new burner technology which produces even less NOx emissions than the low NOx burner systems currently available in the art. The new, extremely low NOx burner would preferably also be less complex, less expensive, more stable, and much simpler to operate, maintain, and control than current low NOx burner systems. Further, the new, extremely low NOx burner would preferably provide a much larger available turndown ratio than is provided by current low NOx burners.
The present invention provides a low NOx burner apparatus and method which satisfy the needs and alleviate the problems discussed above. The inventive burner and method are capable of providing NOx emission levels of 10 parts per million by volume (ppmv) or less (preferably 7 ppmv or less) based on the total volume of combustion gas product produced by the burner. The inventive low NOx burner also provides much stabler operation and is less complex and less costly than the low NOx burner systems currently available. Moreover, the inventive burner is much simpler to maintain and control and provides a desirably broad available operating range. In this regard, the inventive burner has a turndown ration in the range of from about 5:1 to about 10:1, comparable to the broad operating ranges provided by prior high NOx burners.
The inventive burner and method also greatly reduce the need for metal components within the throat of the burner, thus allowing the size of the burner to be significantly reduced. This, coupled with the desirably broad turndown ratio provided by the inventive burner, allows the furnace operator to achieve and maintain more optimum, stable heat flux characteristics and also allows the inventive burner to be more easily sized as needed. Thus, the inventive burner can be conveniently used in existing heaters, boilers, etc. to replace most existing conventional or staged fuel burners with, at most, only minor modifications to the furnace structure.
In one aspect, the present invention involves a burner providing reduced NOx emissions for use in a heating system having a flue gas therein. The burner comprises: (a) a burner wall having a forward end and (b) a series of fuel gas ejectors, each having only a single fuel gas ejection port provided therein. The ejectors and the single fuel gas ejection ports provided therein are positioned and the fuel gas ejection ports are configured to deliver fuel gas from the ejectors in adjacent, free jet flow streams outside of the burner wall toward a combustion zone at the forward end of the burner wall. Each of the adjacent free jet flow streams is effective for entraining a portion of the flue gas in the fuel gas.
In another aspect, the present invention involves a burner providing reduced NOx emissions for use in a heating system having a flue gas therein, the burner comprising: (a) a burner wall having a forward end and (b) a plurality of fuel gas ejectors positioned longitudinally rearward end and laterally outward from the forward end of the burner wall. The burner has only a single combustion stage consisting essentially of a combustion zone beginning at the forward end of the burner wall such that, apart from any pilot fuel employed elsewhere in the burner, substantially all fuel used by the burner is combusted in the combustion zone. The fuel gas ejectors are configured and positioned to deliver fuel gas to the combustion zone in ejection flow paths outside of the burner wall in a manner effective such that at least a sufficient portion of the flue gas is entrained in the fuel gas to yield total NOx emissions of less than 10 parts per million by volume of all combustion product gases produced by the burner.
In yet another aspect, the present invention involves a burner providing reduced NOx emissions comprising: a burner wall having a forward end; at least one fuel gas nozzle positioned longitudinally rearward of and laterally outward from the forward end of the burner wall; at least one fuel gas delivery port positioned in the fuel gas nozzle to deliver fuel gas in a flow path outside of the burner wall to a combustion zone at the forward end of the burner wall; and at least one exterior ledge provided on the burner wall and positioned between the fuel gas delivery port and the forward end of the burner wall such that at least a portion of the fuel gas traveling in the flow path will contact the ledge.
In yet another aspect, the present invention involves a burner providing reduced NOx emissions comprising: a burner wall having a forward end; at least one fuel gas nozzle having at least one fuel gas delivery port positioned therein to deliver fuel gas in a flow path outside of the burner wall to a combustion region at the forward end of the burner wall; and a plurality of impact structures, not including the forward end of the burner wall, positioned such that each of the impact structures will be contacted by at least a portion of the fuel gas traveling in the flow path.
In yet another aspect, the present invention provides a method of reducing NOx emissions from a burner used in a heating system having a flue gas therein, wherein the burner includes a burner wall having an interior passageway with an outlet at the forward end of the burner wall. The method comprises the steps of: (a) ejecting fuel gas outside of the burner wall in free jet flow such that at least a portion of the flue gas is entrained in the fuel gas and the fuel gas travels outside of the burner wall to a combustion zone at the forward end of the burner wall and (b) delivering air or other oxygen-containing gas to the combustion zone via the interior passageway. The burner is operated in the inventive method as a single stage burner wherein, apart from any pilot fuel employed elsewhere in the burner, substantially all fuel used by the burner is combusted in the combustion zone at the forward end of the burner wall.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.